The overall objective of this project is to test the hypothesis that the effects of saturated fat (SF) on lipoprotein markers of cardiovascular disease (CVD) risk are influenced by food sources of dietary protein. There is growing epidemiological evidence that consumption of red meat is associated with greater incidence of CVD than either white meat or non-meat foods. Pathophysiological support for the validity of this association is provided by preliminary evidence from our group that a high beef diet has a more deleterious effect on lipoprotein measures of CVD risk than we have observed for mixed protein diets. Specifically, we have found that a high protein, high SF diet with a moderate red meat content selectively induces increases in intermediate density lipoproteins (IDL) and larger LDL particles that have been found to be much more weakly associated with CVD risk than smaller LDL. In contrast, a more recent study from our group has found that, with a similar intake of SF, high beef consumption results in a preferential increase in levels of small and medium sized LDL particles, both of which are strongly related to incident CVD. To date however, no studies have directly compared the lipoprotein effects of red meats with other food sources of protein in the context of both high and low saturated fat intake. We specifically hypothesize that increases in plasma levels of LDL cholesterol (C), and apolipoprotein (apo) B, induced by SF are greater when the major food source of protein is red meat rather than either white meat (poultry) or non-meat foods, and that this is due to increased levels of small and medium sized LDL particles. We therefore propose a clinical trial in which 180 healthy men and women will be randomized to high SF (15%) or low SF (7%) diet groups, and within each group, consume diets with equivalent amounts of protein derived from red meat, white meat, and non-meat sources for 4 wk each in random order. Our Specific Aims will test whether: (1) with high SF, the red meat diet, compared to the other food sources of protein, will result in higher levels of LDL-C, apoB, small and medium sized LDL particles, and total/HDL-C; (2) with low SF, dietary protein source will not be related to any of these measurements; (3) with both the white meat and non-meat diets, increased LDL-C with high vs. low SF will be due primarily to increases in IDL and/or large LDL, whereas with red meat the additional increase in small and medium LDL will result in greater increases in apoB. In addition to these aims we will test for possible metabolic determinants of dietary effects on apoB-containing lipoprotein subclasses, including post-heparin plasma hepatic lipase activity, which is critical for production of smaller LDL, and LDL receptor activity as assessed in peripheral blood mononuclear cells, a system demonstrated to reflect physiologically relevant LDL receptor regulation. Finally, we will examine potential dietary influences on other metabolic biomarkers of CVD risk, including HDL subclasses and apoproteins, insulin sensitivity as assessed by HOMA-IR, measures of inflammation including CRP and multiple cytokines, and endothelial function using a non-invasive fingertip method.